Specular reflection causes a number of problems in identification, sensing and read-out applications. Identification applications, wherein information is retrieved from data carriers e.g. located on an object to be indentified are widely spread. Current marker techniques, such as for example barcodes or RFID tags do not allow efficient localization of the data carrier on the marked object. In case of barcodes, localization requires complex image processing, while RFID localization may suffer from a poor resolution because of the large wavelength of the radiowaves used. The lack of a good localization possibility limits the use of these techniques. This problem can be overcome by using optical wavelengths or light, e.g. infrared light, for identification applications. Although this requires the objects to be in line of sight, the spatial resolution can be drastically improved as compared to RF based techniques. To identify an object, a photonic integrated circuit (PIC) comprising an identifying signal processing unit or marker chip can e.g. be attached to the object. The marker chip may be flood exposed by use of e.g. a wavelength tuneable laser and its response may be detected by use of e.g. a camera or a detector. Specular reflection of the marker chip makes such components nevertheless useless in remote read-out configurations as the reflected response can only be captured by such a remote control unit in case of perpendicular incidence on the sensor.
A second example for which non-specular transmission or reflection of optical signals is an important feature is in remote read-out of sensors such as for example environmental sensors, control sensors, or monitor sensors. Optical sensors are used in a broad variety of applications. They offer advantages as compared to other types of sensors, including immunity to electro-magnetic interference (EMI), very good stability, long life, small size and low cost. They are especially useful in harsh environments, such as in environments with a high temperature, vibrations, EMI or dust. Optical sensors can be used to measure a wide variety of parameters. Depending on their specific design, specific functionalization or combination thereof, they can be used for measuring for example stress, pressure, strain, torque, vibrations, acoustic waves, temperature, magnetic/electric field, ionizing radiation, biological substances, chemicals, biochemical reactions, drugs, proteins or a combination thereof. Read-out of such sensors, typically performed by exposing the sensors to light and detecting their response, is often difficult as specular reflection results in the impossibility for using a single handheld read-out device unless the read-out occurs perpendicularly with respect to the sensor surface.
To enable optical read-out of optical sensors without the use of on-site optical-electrical conversion, the sensors can be monolithically or hybridly integrated with an optical fiber, or the sensors can be read out remotely by exposing the sensors to light and detecting their response, for example by use of a handheld device combining a light source and a detector. A practicable embodiment allowing the use of a handheld device combining light source and detector is only possible when the optical signal processing units, in casu the optical sensor, enables a response, wherein the light sent back by the sensor to the detector follows substantially the same optical path to the handheld device as the light sent by the handheld device to the sensor.
Several types of optical sensors exist. One type of optical sensor is based on photonic integrated circuits (PICs), also called photonic lightwave circuits (PLCs). PICs (Photonic Integrated Circuits) enable label-free sensing of a multitude of parameters with good signal-to-noise ratio or e.g. good sensitivity. PICs are widely studied for sensing of a multitude of parameters and have been implemented in array configurations to realise micro-array sensor systems. To date, micro-array sensors using PIC technology require stable light incoupling in the PIC waveguides to enable high sensitivity or high signal-to-noise ratio. Coupling is realised by butt-coupling fiber bundles to the PIC. However, this demands for stringent alignment requirements. Moreover, the degree of multiplexing is limited by the physical constraints of fiber bundles. PICs exist in a variety of forms and material systems such as for example low-index contrast waveguide platforms (e.g. polymer waveguides, glass/silica waveguides, AlxGa1-xAs waveguides, InxGa1-xAsyP1-y waveguides), high-index contrast waveguides (e.g. Silicon-on-Insulator, semiconductor membranes), plasmonic waveguides (e.g. metal nano-particle arrays, metal layers). A PIC comprises at least one integrated optical component, such as for example an integrated optical cavity, an integrated optical resonator, an integrated optical interferometer, an integrated optical coupler, a waveguide, a grating or a combination thereof. The optical components can be active or passive. The components can be integrated for example monolithically, heterogeneously or hybridly. Given their small size, PICs and PIC components offer numerous benefits for sensing, such as the potential for the development of miniaturized, compact and robust sensing elements, and prospect of mass fabrication of multiple sensors on one chip. Moreover, because of the opportunity of using very long interaction lengths, PICs and PIC components can show superior sensitivity when compared to bulk optic components. PICs are widely studied for sensing applications, however achieving a tight contact between the environment to be sensed and the sensing element is not easy, especially in remote or in-vivo sensing applications.
A second type of optical sensors are optical fiber sensors. This type of sensors is very suitable for being used in environments that are difficult to access and are therefore the sensors of choice for in-vivo applications. Optical sensors wherein an optical sensing element is provided at a facet of an optical fiber are for example described in U.S. Pat. No. 6,925,213, WO2004/034007, WO2007/019676, Proc. SPIE Vol. 5593, p. 494-501. This type of optical sensors enables a tight contact between the environment to be sensed and the sensing element. However, their performance in terms of sensitivity, flexibility, multidimensional and multi-parameter sensing capability is generally inferior to PIC based optical sensors.